The present invention generally relates to an electromagnetic (“EM”) tracking system. In particular, the present invention relates to a system and method for disambiguating the phase of a field received from a transmitter in an EM tracking system.
Medical practitioners, such as doctors, surgeons, and other medical professionals, often rely upon technology when performing a medical procedure, such as image-guided surgery (“IGS”) or examination. An IGS system may provide positioning and/or orientation (“P&O”) information for the medical instrument with respect to the patient or a reference coordinate system, for example. A medical practitioner may refer to the IGS system to ascertain the P&O of the medical instrument when the instrument is not within the practitioner's line of sight with regard to the patient's anatomy, or with respect to non-visual information relative to the patient. An IGS system may also aid in pre-surgical planning.
The IGS or navigation system allows the medical practitioner to visualize the patient's anatomy and track the P&O of the instrument. The medical practitioner may use the tracking system to determine when the instrument is positioned in a desired location or oriented in a particular direction. The medical practitioner may locate and operate on, or provide therapy to, a desired or injured area while avoiding other structures. Increased precision in locating medical instruments within a patient may provide for a less invasive medical procedure by facilitating improved control over smaller, flexible instruments having less impact on the patient. Improved control and precision with smaller, more refined instruments may also reduce risks associated with more invasive procedures such as open surgery.
In medical and surgical imaging, such as intraoperative or perioperative imaging, images are formed of a region of a patient's body. The images are used to aid in an ongoing procedure with a surgical tool or instrument applied to the patient and tracked in relation to a reference coordinate system formed from the images. Image-guided surgery is of a special utility in surgical procedures such as brain surgery and arthroscopic procedures on the knee, wrist, shoulder or spine, as well as certain types of angiography, cardiac procedures, interventional radiology and biopsies in which x-ray images may be taken to display, correct the P&O of, or otherwise navigate a tool or instrument involved in the procedure.
Generally, image-guided surgery systems operate with an image display which is positioned in a surgeon's field of view and which displays a few panels such as a selected MRI image and several x-ray or fluoroscopic views taken from different angles. In tool navigation systems, the display visible to the surgeon may show an image of a surgical tool, biopsy instrument, pedicle screw, probe or other device projected onto a fluoroscopic image, so that the surgeon may visualize the orientation of the surgical instrument in relation to the imaged patient anatomy. An appropriate reconstructed CT or MRI image, which may correspond to the tracked coordinates of the probe tip, may also be displayed.
Among the systems that have been proposed for effecting such displays, many rely on closely tracking the position and orientation of the surgical instrument in external coordinates. The various sets of coordinates may be defined by robotic mechanical links and encoders, or more usually, are defined by a fixed patient support, two or more receivers such as video cameras which may be fixed to the support, and a plurality of signaling elements attached to a guide or frame on the surgical instrument that enable the position and orientation of the tool with respect to the patient support and camera frame to be automatically determined by triangulation, so that various transformations between respective coordinates may be computed.
The highly accurate tracking technology found in navigation systems may also be used to track the P&O of items other than medical instruments in a variety of applications. That is, a tracking system may be used in other settings where the P&O of an object in an environment is difficult to accurately determine by direct or indirect inspection. For example, tracking technology may be used in forensic or security applications. Retail stores may use tracking technology to prevent theft of merchandise. In such cases, a passive transponder may be located on the merchandise. A transmitter may be strategically located within the retail facility. The transmitter emits an excitation signal at a frequency that is designed to produce a response from a transponder. When merchandise carrying a transponder is located within the transmission range of the transmitter, the transponder produces a response signal that is detected by a receiver. The receiver then determines the location of the transponder based upon characteristics of the response signal.
Tracking systems are also often used in virtual reality systems or simulators. Tracking systems may be used to monitor the position of a person in a simulated environment. A transponder or transponders may be located on a person or object. A transmitter emits an excitation signal and a transponder produces a response signal. A receiver detects the response signal. The signal emitted by the transponder may then be used to monitor the position of a person or object in a simulated environment.
Tracking systems may be optical, ultrasonic, inertial, or electromagnetic, for example. Electromagnetic tracking systems may employ coils as receivers and transmitters. In EM trackers, transmitter coil or coils emit quasi-static magnetic fields. In addition, receiver coil(s) measure these fields. From the field measurements and mathematical models of the coils, the position and orientation of the receiver with respect to the transmitter can be determined. Alternatively, the position and orientation of the transmitter with respect to the receiver is determined.
Typically, an electromagnetic tracking system is configured in an industry-standard coil architecture (“ISCA”). ISCA trackers use a trio of nearly-colocated, nearly-orthogonal, nearly-dipole coils for the transmitter and another trio of nearly-colocated, nearly-orthogonal, nearly-dipole coils for the receiver. Each coil trio is carefully characterized during manufacture to numerically express the precise value of the “nearly” in the previous sentence. From the field measurements and mathematical models of the coils, the position and orientation of the receiver with respect to the transmitter is determined. Alternatively, the position and orientation of the transmitter with respect to the receiver is determined. All six degrees of freedom (three of position and three of orientation) are tracked.
Single-coil EM trackers use a single dipole or nearly-dipole transmitter coil and an array of six or more receiver coils, or else use a single dipole or nearly-dipole receiver coil and an array of six or more transmitter coils. By electromagnetic reciprocity, these two arrangements function equivalently. The coils in the array may be dipole, nearly-dipole, or non-dipole coils (or combinations). The coils in the array are either precisely manufactured or precisely characterized during manufacture to obtain mathematical models of the coils in the array. The single coil does not need to be characterized. From the field measurements and mathematical models, the position and orientation of the single coil with respect to the array are tracked. Since the single coil is symmetrical about its roll axis, only five degrees of freedom (six of position and two of orientation) of position and orientation are tracked. The gain of the single coil can also be tracked.
The array of coils can be fabricated as a printed-circuit board or as an array of wound coils or as a combination of both. Arrangements of coils in the array vary widely in various implementations of single-coil EM trackers. The array may include electrically-conductive or ferromagnetic materials as part of the design of the array.
In an EM system that includes a single-coil transmitter and an array of receiver coils, the tracked outputs typically include position, orientation (without roll information), and gain of the single-coil transmitter. The single-coil transmitter can be made wireless by including a self-contained oscillator in the transmitter that drives the coil to produce a sinewave EM field.
The wireless transmitter coil can be small enough to be approximated as a dipole coil. Such a dipole coil can have a defined dipole magnetic moment vector. For a sinewave excitation, the dipole moment vector is approximated in Equation #1 as:M=M0 sin(2π·F·t)   (1)where M is the dipole moment vector as a function of time, M0 is the dipole moment positive peak value vector, F is the frequency of oscillation of the wireless transmitter coil, and t is time. FIG. 1 illustrates an example graph 100 of the dipole moment vector 130 for a wireless transmitter coil in accordance with an embodiment of the presently described technology. Graph 100 includes a y-axis 110 representative of M as defined above by Equation #1, an x-axis 120 representing various radians, and a curve 130 representative of M at various radians in the above Equation #1. As illustrated in FIG. 1, during a phase defined by 0 to π (or 0° to 90°), the dipole magnetic moment vector transmitted by the wireless transmitter coil is positive. During the phase defined by π to 2π, the dipole magnetic moment vector transmitted by the wireless transmitter coil is negative.
Current EM tracking systems include tracker electronics, or a receiver signal-processing system. The tracker electronics must phase-lock onto the sinewave transmitted by the wireless transmitter. The tracker electronics may then use the sinewave transmitted by the wireless transmitter to determine the position and orientation of the wireless transmitter.
One major difficulty with current systems and methods for tracking positions and orientations of wireless transmitters is confusion between two similar mechanical configurations of a wireless transmitter. Specifically, suppose a wireless transmitter in an EM system is at some position and orientation with respect to one or more receivers. This position and orientation is the first mechanical configuration. Suppose also that the phase-locking hardware or software of the system acquires the signal emitted by the wireless transmitter (referred to as a “tracking signal”). The position and orientation of the transmitter may then be calculated based on this signal. This may be referred to as the first tracked position and orientation. The trackin .g signal emitted by the transmitter may be approximately that shown in FIG. 1.
If the wireless transmitter coil is then rotated end-for-end, without otherwise changing the coil's position or orientation, then the wireless transmitter is placed into a second mechanical configuration. The tracking signal transmitted by the wireless transmitter changes to approximately that shown in FIG. 2. FIG. 2 illustrates an example graph 200 of the dipole moment vector 230 for the wireless transmitter coil in the second mechanical configuration in accordance with an embodiment of the presently described technology. Graph 100 includes a y-axis 110 representative of M as defined above by Equation #1, an x-axis 120 representative of time, and a curve 230 representative of M at various times in the above Equation #1. As illustrated in FIG. 2, during a phase defined by 0 to π (or 0° to 180°), the dipole magnetic moment vector transmitted by the wireless transmitter coil is negative. During the phase defined by π to 2π (or 180° to 360°), the dipole magnetic moment vector transmitted by the wireless transmitter coil is positive.
In the second mechanical configuration, the tracking signal is essentially the same as that for the first mechanical configuration but with the sign of the gain of the transmitter coil changed. In other words, the effect is essentially that of multiplying M0 by −1.
In the second mechanical configuration, phase-locking again occurs. However, current systems and methods phase-lock on the tracking signal of the second mechanical configuration, but out-of-phase. Specifically, current systems and methods lock on to the signal of the second mechanical configuration at 180° (or π) out of phase with respect to the phase-lock on the signal from the first mechanical configuration. The end result is that the two mechanical configurations have the same dipole moment vector M (or the same received mutual-inductance or field measurements). In other words, current systems and methods measure the dipole moment vector of the first mechanical configuration by locking on to the phase starting at 0° (or 0) while locking on to the phase of the second mechanical configuration at 180° (or π). Therefore, current systems and methods determine the same position and orientation for both the first and second mechanical configurations.
Thus, a need exists for a system and method for distinguishing between these two mechanical configurations. Such a system and method disambiguates the sign of the gain of the wireless transmitter coil. Previous systems and methods may cause the transmitter to emit some field at the second harmonic of the sinewave oscillation. The received second harmonic is then used to disambiguate the sign of the gain of the transmitter coil.
However, in some applications, there is insufficient spectrum available to permit dedicating the second harmonic to gain-sign disambiguation. For example, frequencies close to the second harmonic may already be used by other transmitters. In addition, using the second harmonic may not be possible in cases where the transmitter frequency has been made as low as possible to eliminate field distortion due to eddy currents in nearby electrically conductive objects.
Therefore, a need exists for a system and method for disambiguating the phase of a field received from a transmitter in an EM tracking system. Such a system and method permits the accurate tracking of positions and orientations of a transmitter that differ by rotating the transmitter end-for-end.